OFDM is nowadays a mainstream modulation choice for most modern broadband communication systems in use, whether wireline or wireless. The main reason is that OFDM provides the best usage of the available frequency band since it maximizes the spectral efficiency. Moreover, one of other main advantages of OFDM over single carrier modulation is the easy mitigation of inter-symbol interference without having to resort to elaborate equalization, robustness in multipath wireless propagation channels, resilience to impulsive noise and narrowband ingress, and coexistence with legacy services because of spectral compactness and fine frequency granularity.
However, OFDM has a major drawback of having a large peak-to-average power ratio (PAPR). The large PAPR appears as a consequence of the nature of a multicarrier OFDM signal. Namely, when N modulated tones of the OFDM signal add together, the peak magnitude might have a value of N or even larger at a certain point in time if all tones add constructively, while the average might be quite low due to interferences between all modulated tones with independent amplitudes and phases.
The PAPR issue in OFDM systems can be simply understood by considering the example of an M-ary PSK modulation per tone. In this case, there are at most M2 patterns amongst MN that yield the highest PAPR, namely, N.
PAPR values for N=2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048 and QPSK modulation per tone is listed in the following Table 1. The PAPR value increases by 3 dB per octave in the number of tones (i.e., each time we double the number of tones).
TABLE 1PAPR values for several typical values of N when a QPSK modulation isapplied to each tone.N24816326412825651210242048PAPR3 dB6 dB9 dB12 dB15 dB18 dB21 dB24 dB27 dB30 dB33 dB
To decrease the power consumption and to prolong the battery life and save the system power, we need to seek to lower PAPR significantly.
A high PAPR impacts the implementation complexity and the power dissipation. Numerous approaches utilized to reduce the PAPR value have been proposed during the previous two to three decades and are still under thorough studies.
Among the existing PAPR-reduction approaches, the Selective Mapping (SLM) method is considered as one of the most popular and effective approaches. According to the document <<A Method to Reduce the Probability of Clipping in DMT-Based Transceivers>> by Denis J. G. Mestdagh and Paul M. P. Spruyt (IEEE Transactions on Communications, vol. 44, n. 10, pp. 1234-1238, October 1996), the SLM method comprising at steps of generating L statistically independent input data sequences which represent the same information, processing each input data sequence by utilizing L parallel N-IFFT to generate L time-domain OFDM symbols, and selecting, among the time-domain OFDM symbols, one symbol with the smallest PAPR for transmission.
The key point of the SLM method lies in how to generate multiple distinct time-domain OFDM symbols when the input data for transmission is the same. For this purpose, L pseudo-random phase rotation sequences Ψl=[Ψl,0 Ψl,1 . . . Ψl,N-1]T with l=1, 2, . . . , L are defined; where Ψl,k=ejψl,k and ψl,k is uniformly distributed in [0, 2π]. This process can be seen as performing a dot product operation on the input tones X(k) with rotation factor ejψl,k. In practice, all the elements of the phase sequence Ψ1 are set to 1 to as to make this branch sequence the original OFDM symbol. The complexity of the original SLM is usually too high to be acceptable in actual implementations. In order to reduce the complexity of the conventional SLM scheme, several modified SLM methods using conversion matrices that can relieve the requirement of multiple N-IFFTs have been proposed.
The most interesting one is the so-called <<TSCM-SLM>> method, which is described in a document <<A Low-Complexity SLM Approach Based on Time-domain Sub-block Conversion Matrices for PAPR Reduction>> by Y.-R. Tsai, C.-H. Lin and Y.-C. Chen (IEEE Symposium on Computers and Communications (ISCC), pp. 579-584, June 2011). Nevertheless, in order to significantly reduce the computational complexity, the independancy of the available candidate OFDM signals is compromised and the effectiveness of PAPR reduction according to the TSMC-SLM method is degraded. For example, as shown in FIGS. 6 and 7 of the above-mentioned document, the conventional SLM method provides a greater PAPR reduction than the TSCM-SLM method. In the example of N being 256, the degradation is about 0.9 dB.
It should be noted, however, that none of the known approaches provide a global solution concerning the desired (or mandatory) features and/or advantages listed here below:                Very high PAPR value and its complementary cumulative distribution function (CCDF(PAPR)),        Deterministic PAPR and CCDF(PAPR) reduction capability,        No iteration required at the transmitter nor at the receiver,        Low to very-low implementation complexity at both of the transmitter and the receiver,        No loss of capacity,        No need to increase the transmit power,        No need of transmitter to receiver side information,        No bandwidth expansion,        Neither in-band distortion nor out-of-band leakage,        Backward compatible with a conventional-OFDM mode of operation.        
Actually, there is a big need for a holistic approach by considering the sum of the analogue and the digital transceiver's power consumption. Indeed, it would be useless to implement complex and heavy-computational-load PAPR reduction digital methods whose effects would lead to a situation where the contributions to the power consumption are changing fundamentally in a way that the digital signal processing becomes the dominant part of the overall consumption. While scaling of silicon-based processor efficiency with the help of Moore's law is approaching physical limits, the wireless communications need a new approach to significantly reduce the PAPR value while performing minimal low power signal processing computations. This is actually what the present invention is all about. In addition, according to the present invention, the system is more robust against Doppler effects and provides a means to increase the secrecy at the physical-layer of the communication system.
Reducing the PAPR value, and in the best case minimizing it, allows a power consumption reduction of the power amplifier and the digital-to-analog converter(s) (DAC(s)) when the average signal power must be kept fixed. On the other hand, higher average signal power can be transmitted for a fixed amplifier power supply PDC and thus improving the overall signal-to-noise ratio (SNR), and consequently the BER at the receiver, allowing a larger wireless coverage from, for example, the base stations of broadcasters or base stations of cellular mobile network operators.